1. Field of the Invention
The present invention relates to an image forming apparatus employing, in general, an electrophotographic printing method, such as a copying machine and a printer.
2. Description of the Related Art
Conventionally, as image forming apparatus employing an electrophotographic printing method, there have been known intermediate transfer type image forming apparatus, in which a toner image formed on an electrophotographic photosensitive member (photosensitive drum) is primarily transferred onto an intermediate transfer member, and the toner image is secondarily transferred onto a recording material (transfer material) to be output.
Of those, various types of in-line type image forming apparatus have been proposed because the in-line type image forming apparatus can form an image at high speed. In the apparatus of this type, a plurality of image forming units configured to form toner images of different colors are arrayed in series in a moving direction of a transfer belt serving as a recording material conveying member (transfer material carrying member) or an intermediate transfer belt serving as an intermediate transfer member. In this configuration, the toner images are sequentially transferred in a superimposed manner from a plurality of photosensitive drums onto the recording material or the intermediate transfer belt.
In such an image forming apparatus, when toner remains or adheres on the surface of the transfer belt or the intermediate transfer belt, cleaning is somehow required because residual toner causes image failure. In order to remove the toner (un-transferred residual toner) remaining or adhering to the surface of the transfer belt or the intermediate transfer belt, there is provided a cleaning device such as a blade which is brought into contact with the transfer belt or the intermediate transfer belt to scrape off the toner. Further, there is proposed a cleaning device which is configured to collect the scraped unnecessary toner into a waste toner tank.
Further, there is also proposed a system in which the above-mentioned cleaning device is not provided, but an un-transferred residual toner collecting electric field is formed at a primary transfer portion so that the un-transferred residual toner on the surface of the transfer belt or the surface of the intermediate transfer belt is electrostatically and reversely transferred onto the photosensitive drum, and the un-transferred residual toner is collected by a cleaning device provided for the photosensitive drum.
In this case, even when the normal charging polarity of the toner is negative (minus), because the un-transferred residual toner is fogging toner or un-secondarily-transferred residual toner, negatively-charged toner and positively-charged (plus) toner are present. Therefore, as for the un-transferred residual toner collecting electric field formed at the primary transfer portion, both of a “transfer electric field” in the same direction as that during image formation and a “reverse transfer electric field” in a direction opposite thereto are necessary.
That is, when the negatively-charged un-transferred residual toner is collected, an exposure unit controls a photosensitive drum surface potential to be in the vicinity of 0 V, and a voltage which is larger on the negative side than that of the photosensitive drum surface is applied to a primary transfer unit. With this, the “reverse transfer electric field” in the direction opposite to that during normal image formation is formed at the primary transfer portion.
On the other hand, when the positively-charged un-transferred residual toner is collected, the photosensitive drum surface is charged to have a negative potential similar to the case during normal image formation, and a positive transfer voltage is applied to the primary transfer unit. With this, the “transfer electric field” in the same direction as the transfer electric field during normal image formation is formed at the primary transfer portion.
Further, there is proposed a method of effectively removing the un-transferred residual toner in the in-line image forming apparatus including a plurality of photosensitive drums to form a plurality of primary transfer portions (Japanese Patent Application Laid-Open No. 2010-117730). In this method, collection is controlled by distinguishing photosensitive drums for collecting the negatively-charged un-transferred residual toner and photosensitive drums for collecting the positively-charged un-transferred residual toner.
Further, it is also proposed in this proposal that, in order to prevent bias in amount of toner to be collected in waste toner containers provided to the respective photosensitive drums, a detection device such as a sensor is provided to the waste toner container, and control of performing distributing collection of the un-transferred residual toner is made in accordance with the detection results. In this case, a control of causing the positively-charged un-transferred residual toner to pass through a predetermined photosensitive drum without being collected is also necessary. In this case, the photosensitive drum surface is exposed with light to form the “reverse transfer electric field” in the direction opposite to that during normal image formation at the primary transfer portion.
Further, a charging device configured to charge the un-transferred residual toner is provided so that the un-transferred residual toner is charged to a predetermined polarity in advance. In this manner, the amount of toner to be reversely transferred onto each photosensitive drum can be controlled and grasped more accurately.